A frame structure in the TDD mode of the long term evolution (LTE for short) system is shown in FIG. 1. In such a frame structure, one radio frame with a length of 10 ms (307200 Ts, 1 ms=30720 Ts) is divided into 2 half-frames each of which is 5 ms (153600 Ts) long and comprises 8 normal time slots with a length of 0.5 ms and 3 special time slots. The 3 special time slots are a downlink pilot time Slot (DwPTS for short), a guard period (GP for short), and an uplink pilot time slot (UpPTS for short), and the total lengths of these 3 special time slots is 1 ms (30720 Ts). Subframe 1 is always consisted of 3 special time slots; when there are 2 downlink-to-uplink switching points in 10 ms, subframe 6 is consisted of 3 special time slots; in other cases, subframe 6 only comprises a DwPTS (here the length of the DwPTS is 1 ms). Other subframes are consisted of 2 normal time slots.
In the above frame structure, subframes 0, 5 and the DwPTS are always used for downlink transmission, and subframe 2 and the UpPTS are always used for uplink transmission. When there are 2 downlink-to-uplink switching points in 10 ms, subframe 7 is also used for uplink transmission.
In the TDD mode of the LTE system, a physical random access channel (PRACH for short) has two categories:
The first category: this category of the PRACH is transmitted in normal uplink subframes (subframes with special time slots are not included), and this category of the PRACH has four types of formats:
(1) Preamble format 0: occupying 1 uplink subframe, the length of the cyclic prefix (CP for short) is 3168 TS and the length of the preamble is 24576 Ts;
(2) Preamble format 1: occupying 2 uplink subframes, the length of the CP is 21024 TS, and the length of the preamble is 24576 Ts;
(3) Preamble format 2: occupying 2 uplink subframes, the length of the CP is 6240 TS and the length of the preamble is 2×24576 Ts;
(4) Preamble format 3: occupying 3 uplink subframes, the length of the CP is 21024 Ts and the length of the preamble is 2×24576 Ts;
The second category: this category of the PRACH is transmitted in the UpPTS, and this category of the PRACH has one type of format:
(1) Preamble format 4: the length of the CP is 448 Ts and the length of the preamble is 4096 Ts.
In the frequency domain, the various PRACHs mentioned above all occupy 6 resource blocks (RB for short), each RB comprises 12 subcarriers, and the bandwidth of each subcarrier is 15 kHz.
When a mobile terminal such as a cell phone accesses a system, downlink synchronization is performed first, and then the cell phone demodulates the broadcast channel to obtain the configuration parameters of the PRACH, and finally uplink synchronization is accomplished via the PRACH, and a connection to a base station is established. Herein, the configuration parameters of the PRACH in the TDD mode comprises density (viz. the number of the available PRACHs per time unit), preamble format (hereinafter referred to as format for shot), and version number. Wherein, if the formats and densities are the same but the versions are different, it means that the preamble formats are the same and the numbers of the PRACHs per time unit are the same, while the locations of these PRACHs in the time domain or in the frequency domain are different. In a practical application, a plurality of versions can be set for the PRACHs with the same format and the same density. The purpose for using these different versions of the PRACHs in different cells is: to scatter the PRACHs of different cells managed by the same base station in the time domain for trying to allow the respective cells managed by the same base station to initiate a PRACH processing request at different times so as to avoid the case that the base station is over busy at some time, while it has no data to process at other time. In addition, for the PRACH of the preamble format 4, since data are not sent in the UpPTS, different cells use different versions, the PRACH of each cell has a different location in the time domain or in the frequency domain, reducing the inter-cell interference of the PRACHs.
How to indicate the PRACH configuration parameters to a terminal with less air interface resources is a problem to be urgently solved.